BIOLOGY OF ALEURODISCUS 
449 
nutrient agar for a time and allowing the spores to fall upon the agar. 
The mycelium of A. amorphus has never produced fruit bodies in these 
cultures. Its spores germinate only occasionally in water, and the various 
decoctions used, including those of Abies bark, have been ineffective for 
inducing germination. The complete life cycle of the plant has therefore 
not been traced, nor is it known how infection of the fallen Abies trees 
takes place. The mycelium of the fungus is found growing through the 
bark tissues and the cambium. The mycelium remains intercellular. It 
never enters the phellogenous tissues, and when the other elements of the 
bark are disintegrated the corky layer remains intact. It is evident as a 
lightly stained layer, several cells deep, just beneath the epidermis of the 
bark in figures i to 7. In certain areas the mycelium concentrates and 
forms a cushion-like stroma of pseudoparenchymatous tissue in the bark 
parenchyma. The hyphae are thicker here than are those that ramify 
among the elements of the bark, and they branch frequently. As this 
stroma grows, the surface of the bark is arched upward to form a superficial 
prominence (figs. 1,2). There now sets in a strong upward growth of the 
upper stromal hyphae and the pressure produces a rupture in the bark by 
which the rapidly elongating hyphae emerge (fig. 2). They branch fre- 
quently and continue growth until they attain the form represented in 
figures 3-5. 
The hymenium now begins its formation in the even upper surface. 
In figure 6 the hymenium is shown in the process of development. The 
marginal hyphae are longer and more slender from the first, so that under 
a hand lens they present the appearance of a white hairy margin. Occa- 
sionally, in some fruit bodies, these hyphae grow much more rapidly than 
do the central elements of the fruit body, with the result that the structure 
is decidedly concave, instead of convex as is typically the case (fig. 7). In 
some cases, the marginal hyphae have grown so rapidly as to arch over 
almost completely the young hymenium, so that in sections of these fruit 
bodies made somewhat tangentially they appear altogether gymnocarp 
because of the perithecioid form of the structure. This latter method of 
development the writer attributes to dry conditions of growth, as it is 
usually encountered on the drier branches of the firs. 
In the hymenium the most conspicuous elements are the large basidia 
with their prominent fusion nuclei. Beside them are the nodulose or monili- 
form paraphyses and the more slender filiform paraphyses. The basidia 
keep pace uniformly in their growth so as to present an even, level 
peripheral surface. The paraphyses project usually about 25 /jl above the 
general level. When a basidium begins putting out sterigmata it elongates 
so as to stand some 25 fx above the general level of the younger basidia. 
These points are brought out in the photomicrograph (fig. 12), in which 
one old basidium with sterigmata is shown elongatfed so as to attain the 
general level of the paraphyses. In thin sections only two of the four 
